This invention relates to naphthyloxyalkyl(meth)acrylate compounds with a high refractive index and whose respective homopolymer has a low glass transition temperature.
Adhesives or coatings used in optical applications preferably have a refractive index closely matched to the refractive index of a substrate to which they are applied. The matched refractive indices reduce glare and reflectance at the interface between the substrate and coating materials and enhance the optical performance of the construction.
Polymeric substrate materials typically used in optical applications have refractive indices of about 1.48 to about 1.65, such as, for example, polymethyl(meth)acrylate (1.489), polycarbonate (1.585), and polyethylene terephthalate (1.64). However, many polymeric coatings and adhesives intended for application to these substrates have refractive indices that differ substantially from the refractive indices of the substrates. The mismatched refractive indices may cause glare and reflectance at the interface between the materials.
Reactive chemical monomers can be used, alone or in combination with other materials, to produce adhesives and coatings with high indices of refraction. However, these adhesive and coating compositions may have other less desirable physical and chemical properties. For example, certain naphthalene derivatives of Formula 1 below have an index of refraction of about 1.57-1.58: 
wherein R1 is H or CH3, R2 is CH2CH2O or CH(CH3)CH2O, X is H, and m is 1. While the refractive index of the monomers in Formula 1 may match well with polymeric substrates commonly used in optical applications, homopolymers made from known compounds of this type typically have a glass transition temperature (Tg) of about 25 to about 40xc2x0 C. For this reason, polymers including these monomeric units would be expected to be essentially non-tacky at or near room temperature (about 20 to about 30xc2x0 C.), which limits their usefulness in optical adhesives and flexible coatings.
In one aspect, the invention is a naphthyloxyalkyl(meth)acrylate monomer with an index of refraction of greater than about 1.55 and a glass transition temperature of its respective homopolymer of less than about 10xc2x0 C. The naphthyloxyalkyl(meth)acrylate monomers include a (meth)acrylate functional group and a naphthyloxy functional group linked by an alkyl functional group.
The preferred naphthyloxyalkyl(meth)acrylate monomers of the invention have the general Formula 2: 
wherein L is a straight chain or branched alkyl group containing greater than 5 carbon atoms, preferably from about 6 to about 8 carbon atoms, and R is H or CH3.
In a second aspect, the invention is a polymerizable composition containing the monomer of Formula 2. The polymerizable composition can contain one or more other compatible comonomers.
In a third aspect, the invention is a polymer or polymeric material including a chemical segment of Formula 3: 
wherein L is a straight chain or branched alkyl group containing greater than 5 carbon atoms, preferably from about 6 to about 8 carbon atoms, and R1 is H or CH3. The polymers with the monomeric segment of Formula 3 have a low glass transition temperature of less than about 10xc2x0 C., and are well suited for use in optical adhesives and flexible coating compositions. The adhesives and coating compositions made from the monomers of the invention would be expected to reduce glare and reflectance when applied to a polymeric substrate with a similarly high refractive index.
In a fourth aspect, the invention is an optical element including a composition with the polymer or polymeric material with the segment of Formula 3.
In a fifth aspect, the invention is an optical device including a composition with the polymer or polymeric material with the segment of Formula 3.
In a sixth aspect, the invention is a method for reducing glare and/or reflectance on a substrate that includes coating the substrate with a composition with the polymer or polymeric material with the segment of Formula 3.
The details of one or more embodiments of the invention are set forth in the description below. Other features, objects, and advantages of the invention will be apparent from the description and from the claims.
The invention is a naphthyloxyalkyl(meth)acrylate monomer with a high refractive index and whose respective homopolymer has a low glass transition temperature. The monomers have specific index of refraction, melting point, and viscosity properties that are particularly suited for use in polymeric materials intended for application to a substrate in an optical element or an optical device.
As used within the present description, xe2x80x9cmonomerxe2x80x9d refers to a monomer on an individual (i.e., molecular) scale, and also to a composition of such monomers on a macroscopic scale such that the composition can be described as having a physical state of matter (e.g., liquid, solid, etc.) and physical properties (e.g., melting point, viscosity, glass transition temperature (of a polymeric form), and index of refraction).
The naphthyloxyalkyl(meth)acrylate monomers of the invention have an index of refraction of at least about 1.54, preferably between about 1.54 and 1.56. xe2x80x9cIndex of refraction,xe2x80x9d or xe2x80x9crefractive index,xe2x80x9d refers to the absolute refractive index of a material (e.g., a monomer), which is understood to be the ratio of the speed of electromagnetic radiation in free space to the speed of the radiation in that material, with the radiation being of sodium yellow light at a wavelength of about 583.9 nanometers (nm). Index of refraction can be measured by known methods, and is generally measured using an Abbe Refractometer.
Homopolymers of the naphthyloxyalkyl(meth)acrylate monomers of the invention have a Tg below about 10xc2x0 C., preferably less than about 7xc2x0 C., and most preferably less than about 5xc2x0 C. xe2x80x9cGlass transition temperature,xe2x80x9d (Tg), is the temperature range over which a thermoplastic polymer changes from a brittle, glass state to a plastic state. Glass transition temperature of a composition can be measured by methods known in the art, such as Differential Scanning Calorimetry (DSC), modulated DSC (MDSC), or Dynamic Mechanical Analysis (DMA).
The naphthyloxyalkyl(meth)acrylate monomers include a (meth)acrylate functional group and a naphthyloxy functional group linked by an alkyl functional group.
The alkyl group is a divalent organic hydrocarbon group. The alkyl group, which is preferably unsubstituted, can be straight or branched and includes greater than 5 carbon atoms, more preferably from 6 to 8 carbon atoms. The size of the alkyl group can affect the physical properties of the monomer and a polymer prepared from the monomer including, for example, the refractive index of the monomer and the refractive index and glass transition temperature of a polymer prepared from the monomer. While not wishing to be bound by any theory, a relatively larger alkyl group may result in a monomer or a polymer with a relatively lower index of refraction compared to an otherwise similar monomer or polymer having a relatively smaller alkyl group. Again, while not wishing to be bound by any theory, relatively larger or more branched alkyl groups may provide a monomer which, when polymerized, has a relatively lower Tg compared to a polymer prepared from otherwise similar monomers having relatively smaller or less branched alkyl groups.
The naphthyloxy group in the monomer of the invention is also preferably unsubstituted.
The (meth)acrylate group in the monomer of the invention can be prepared by methods well known to those skilled in the art. The naphthyloxy alkanol intermediates can be esterified with (meth)acyloyl chloride using a suitable base. They can also be prepared by condensation with (meth)acrylic acid, or by transesterification using, for instance, methyl (meth)acylate. Suitable inhibitors can be added to the reactions during the synthesis of the monomers to prevent premature polymerization of the monomers.
Examples of useful naphthyloxy alkyl acrylate monomers include those with the structure of Formula 2: 
wherein L is a straight chain or branched alkyl group containing greater than 5 carbon atoms, preferably from about 6 to about 8 carbon atoms, and R is H or CH3.
Examples of useful monomers include 6-(1-naphthyloxy)-1-hexylacrylate, wherein the oxyalkylacrylate group is located at the 1 or xcex1 position on the naphthalene ring structure and the alkyl unit has 6 carbon atoms. This monomer, shown below in Formula 4, has a refractive index of about 1.556 and the Tg of its homopolymer is about 3xc2x0 C. 
Another preferred monomer is 6-(2-naphthyloxy)-1-hexylacrylate, wherein the oxyalkylacrylate group is located at the 2 or xcex2 position on the naphthalene ring structure and the alkyl unit has 6 carbon atoms. This monomer, shown below in Formula 5, has a refractive index of about 1.553 and the Tg of its homopolymer is about 6xc2x0 C. 
Another preferred monomer is 8-(1-naphthyloxy)-1-octylacrylate, wherein the oxyalkylacrylate group is located at the 1 or xcex1 position on the naphthalene ring structure and the alkyl unit has 8 carbon atoms. This monomer, shown below in Formula 6, has a refractive index of about 1.547 and the Tg of its homopolymer is about xe2x88x9249xc2x0 C. 
The monomer of the invention, alone or in combination with materials such as other unsaturated polymerizable comonomers, can be included in a polymerizable composition that can be polymerized or co-polymerized to produce useful polymers or copolymers. As used within the present description, the term xe2x80x9cpolymerizablexe2x80x9d refers to chemical compounds such as monomers and oligomers, etc., and chemical compositions, capable of polymerizing or copolymerizing (e.g., via unsaturated moieties) to produce a higher molecular weight materials such as an oligomer, polymer, prepolymer, or polymeric material. The terms xe2x80x9cpolymerxe2x80x9d and xe2x80x9cpolymeric materialxe2x80x9d are used interchangeably to refer to materials prepared from the reaction of one or more polymerizable materials, e.g., one or more polymerizable monomer, oligomer, polymer, or prepolymer, etc., to produce a dimer, trimer, oligomer, copolymer, homopolymers, etc.
The polymers and polymeric materials include a chemical segment of Formula 3: 
wherein L is a straight chain or branched alkyl group, preferably containing greater than 5 carbon atoms, preferably from about 6 to about 8 carbon atoms, and R1 is H or CH3.
Useful comonomers to be reacted with acrylic monomers such as the monomer described herein are known in the organic chemistry art, and can include any of a number of known and useful polymerizable moieties, e.g., vinyl, (meth)acrylate, N-vinyl, acrylic acid, methacrylic acid, allyl, acrylamide, acrylonitrile, etc. The comonomer can be mono- or multifunctional with respect to the unsaturated moiety, and where multifunctional, the unsaturated moieties need not be of identical chemistry.
Specific types of comonomers useful in the polymerizable composition can include (meth)acrylate-functional comonomers such as butyl (meth)acrylate, as well as vinyl comonomers such as methyl styrene. The particular comonomers included in any given polymerizable composition, their molecular weight or weights, and the included amounts of each, can be chosen according to various factors such as the desired nature and properties of the polymerizable composition and the desired properties of the polymer or polymeric material to be prepared therefrom (e.g., index of refraction, glass transition temperature, melting point, viscosity, etc., of the polymerizable composition or polymeric material).
The polymerizable composition can also contain other ingredients that, as will be appreciated by those skilled in the art of polymeric materials, can be useful in such a polymerizable composition. For example, the polymerizable composition might contain a crosslinking agent, one or more surfactants, pigments, fillers, polymerization inhibitors, or other ingredients that can be useful within a polymerizable composition or an optical product. Such ingredients can be included in the composition in amounts known to be effective for their respective purposes.
A crosslinking agent can be useful to increase the glass transition temperature of the polymer resulting from crosslinking the polymerizable composition.
Polymeric beads, inorganic fillers, and/or pigments can be added to the polymerizable composition in order to improve processing, to impart slip and scratch resistance to the polymerized material, or to affect optical properties of the polymerized material. Examples of useful polymeric beads include those made of polystyrene, polyacrylates, copolymers of styrene and acrylates, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof. Examples of inorganic fillers and pigments include solid or hollow glass beads, silica, zirconia, aluminum trihydroxide, and titanium dioxide.
Polymerization of the polymerizable composition can be accomplished by known and usual means, such as heating in the presence of a free-radical initiator, irradiation with electromagnetic radiation such as ultraviolet or visible light in the presence of suitable photoinitiators, and by electron beam.
Polymeric materials (i.e., homopolymers or copolymers) prepared from the monomer of the invention have a Tg below about 10xc2x0 C., as well as a refractive index greater than about 1.54. This makes the polymers particularly well suited to use as coatings and adhesives on polymeric substrates used in optical elements and optical devices, such as, for example, polymethylmethacrylate (PMMA), polycarbonate (PC), and polyethyleneterephthalate (PET). For example, as described in copending U.S. application Ser. No. 09/605,500 incorporated by reference, the polymers prepared from the monomers of the invention may be used to make pressure-sensitive adhesives to be applied to a substrate in an optical device. Polymers prepared from the monomers of the invention may also be used as a coating for an optical element in an optical device.
Compared to conventional polymers, the polymers made from the monomers of the invention have refractive indices that are more closely matched with the refractive indices of polymeric substrates commonly used in optical elements and devices. This refractive index match reduces glare and reflectance at any optical interface between the substrate and the polymers, so the polymers made from the monomers of the invention may be used as part of a method for reducing glare and/or reflectance on a substrate. The method includes coating the substrate with a composition with the polymer or polymeric material with the fragment of Formula 3.
The term xe2x80x9cglarexe2x80x9d as used herein means the average reflectance over a range of about 400 to about 700 nm. The term xe2x80x9creflectancexe2x80x9d means the fraction of incident flux on a surface that is returned to into the same hemisphere whose base is the surface and which contains the incident radiation.